1. Field of the Invention
The present invention relates generally to circuit breakers and, more particularly, to a multi-pole circuit breaker having a mutual connection connecting the poles with one another at a connection point disposed between the separable contacts and the trip unit of each pole in order to equalize the current that flows through a given point of each pole and that is sensed by the trip unit.
2. Description of the Related Art
Electrical switching apparatus for electric power distribution systems includes circuit breakers and network protectors which provide protection, and electrical switches for isolating parts of the distribution system and for transferring between alternative sources. While families of such switches are produced having a range of current ratings, some applications require higher current ratings than are available from the standard units. It is not practical to make a dedicated switch for such applications in view of the limited demand. It is therefore common to mount a pair of such switches side-by-side and to connect the poles to share the current. It is similarly common to provide a multi-pole circuit breaker and connect the poles thereof together in parallel to similarly share the current. Such a parallel construction technique is of particular value with molded case switches where the required investment in the molded case is quite large and can be alleviated by adjoining multiple circuit breakers together or by combining multiple poles of a multi-pole circuit breaker together. Such parallel circuit breaker configurations have not, however, been without limitation.
For instance, in the example in which the poles of a multi-pole circuit breaker have been connected with one another in parallel, such circuit breakers typically have a single operating mechanism that substantially simultaneously separates the separable contacts of each of the poles to interrupt current flowing through the poles during certain specified conditions. It is understood, however, that such operating mechanisms do not separate all of the sets of separable contacts in a precisely simultaneous fashion. More particularly, it is typically the case that the separable contacts of one particular pole of a multi-pole circuit breaker are invariably the last contacts to separate during operation of the circuit breaker, and the time lag after which the separable contacts of the particular pole separate may be only a fraction of a second. Such a time lag can result from numerous factors, including manufacturing tolerances and imprecision, wear, and other factors.
In a situation in which multiple poles of a circuit breaker are connected with one another in parallel, and during the time lag while cycling the operating mechanism in which all of the sets but one of the separable contacts are separated, all of the current that had been flowing through the multiple poles seeks to travel through the single pole whose separable contacts are still connected with one another. When this last set of separable contacts actually separates, an electrical arc larger than any of the arcs formed across the other sets of separable contacts extends across the final set of opening contacts. Such a large arc has the effect of degrading the contacts due to vaporization of the material of the contacts and other factors. Such degradation of the contacts of the pole reduces the amount of current that can be carried through the pole, such that once the circuit breaker is returned to operation, the poles have an unequal current carrying capability with the result that greater amounts of current travel through some poles than through others.
Each pole of such multi-pole circuit breakers typically includes a trip unit that senses the current flowing through a given point of a conductor of the pole. Such multi-pole circuit breakers additionally include a single operating mechanism that is common to all of the poles and that is operative to separate the sets of separable contacts of the poles. Any of the trip units of the circuit breaker can trigger the operating mechanism to interrupt current flowing through all of the poles when the trip unit detects an overcurrent or under-voltage condition or other condition that warrants a trip operation.
In a situation in which one or more of the sets of separable contacts have experienced some degradation such that each of the poles is carrying different amounts of current therethrough, it is possible that the trip unit connected with the highest current-carrying pole may cause the operating mechanism to trip the entire circuit breaker even though the aggregate current carrying capability of the circuit breaker has not been reached. In effect, therefore, a single trip unit can trip the entire circuit breaker even though neither the circuit breaker on an aggregate basis nor the load is experiencing a condition that would warrant the circuit breaker to trip.
It is thus desired to provide a multi-pole circuit breaker in which the poles thereof are connected in parallel in such a fashion to alleviate the risk of unintended tripping based upon unequal current flow through the pole. It is preferred that such an improved multi-pole circuit breaker include a mutual connection that connects together the poles in such a fashion that the trip units of the poles are isolated from the separable contacts and are not subjected to unequal current flowing through the poles.
In view of the foregoing, a circuit breaker includes a plurality of poles that are connected with one another in parallel, and additionally includes a mutual connection connecting the poles with one another to isolate the trip units associated with the poles from any imbalance in the current flowing through the poles. Each pole includes a first conductor structured to be connected with a power source, a second conductor structured to be connected with an electrical load, and a pair of separable contacts that disconnectably connect the first conductor with the second conductor. The trip unit of each pole is configured to measure the current flowing through a given point of the second conductor, and the mutual connection connects the second conductors together at a connection point, the connection point of each second conductor being between the separable contacts and the given point.
An aspect of the present invention is to provide a multi-pole circuit breaker having the poles thereof connected with one another in parallel in such a fashion to alleviate the likelihood of undesired tripping of the circuit breaker due to an imbalance in the current flowing through the different poles of the circuit breaker.
Another aspect of the present invention is to provide a multi-pole circuit breaker having an aggregate load-carrying capacity that is substantially unaffected by repeated cycling of the circuit breaker.
Another aspect of the present invention is to provide a multi-pole circuit breaker in which the poles are connected with one another in parallel, and which additionally includes a mutual connection connecting the poles together to substantially equalize the current flowing through the poles at the points where the current flow through each pole is sensed by a trip unit.
Another aspect of the present invention is to provide a circuit breaker, the general nature of which can be stated as including a plurality of poles, each of the poles including a first conductor, a first contact, a second contact, a second conductor, and a trip unit, the first contact being electrically connected with the first conductor, the second contact being electrically connected with the second conductor, the first and second contacts being separably electrically conductively engaged with one another, the trip unit being responsive to current flowing through a given point of the second conductor, each second conductor including a connection point disposed between the second contact and the given point, the first conductors being structured to be connected with a power source, and the second conductors being structured to be connected with an electrical load, an operating mechanism structured to separate the first electrical contacts from the second electrical contacts responsive to the trip units, and an equalization system including a mutual connection that connects together the connection points of the second conductors.
Another aspect of the present invention is to provide an apparatus for use in a multiple pole circuit breaker, each pole including a first conductor, a first contact, a second contact, a second conductor, and a trip unit, the first contact being electrically connected with the first conductor, the second contact being electrically connected with the second conductor, the first and second contacts being separably electrically conductively engaged with one another, the trip unit being responsive to current flowing through a given point of the second conductor, and each second conductor including a connection point disposed between the second contact and the given point, in which the general nature of the apparatus can be stated as including an equalization system including a mutual connection that connects together the connection points of the second conductors.